1. Field of the Invention
This invention relates to drawn polyethylene multi-filament yarns and articles constructed therefrom. The drawn yarns and articles are useful in applications requiring impact absorption and ballistic resistance, such as body armor, helmets, breast plates, helicopter seats, spall shields; composite sports equipment such as kayaks, canoes, bicycles and boats and in fishing line, sails, ropes, sutures and fabrics.
2. Description of the Related Art
Multi-filament “gel spun” ultra-high molecular weight polyethylene (UHMWPE) yarns are produced by a number of companies, including Honeywell International Inc., DSM N.V., Toyobo Co., Ltd., Ningbo Dacheng and Tongyizhong Specialty Fibre Technology and Development Co., Ltd. Gel-spun polyethylene fibers are prepared by spinning a solution of UHMWPE into solution filaments, cooling the solution filaments to a gel state, then removing the spinning solvent. One or more of the solution filaments, the gel filaments and the solvent-free filaments are drawn to a highly oriented state. The gel-spinning process discourages the formation of folded chain lamellae and favors formation of extended chain structures that more efficiently transmit tensile loads.
The first description of the preparation and drawing of UHMWPE filaments in the gel state was by P. Smith, P. J. Lemstra, B. Kalb and A. J. Pennings, Poly. Bull., 1, 731 (1979). Single filaments were spun from 2 wt. % solution in decalin, cooled to a gel state and then stretched while evaporating the decalin in a hot air oven at 100 to 140° C.
More recent processes [see, e.g., U.S. Pat. Nos. 4,551,296, 4,663,101, and 6,448,659] describe drawing all three of the solution filaments, the gel filaments and the solvent-free filaments. A process for drawing high molecular weight polyethylene fibers is described in U.S. Pat. No. 5,741,451. Yet more recent drawing processes are described in co-pending U.S. application Ser. No. 10/934,675 and in United States Publication 20050093200. The disclosures of U.S. Pat. Nos. 4,551,296, 4,663,101, 5,741,451 and 6,448,659, U.S. application Ser. No. 10/934,675 and United States Publication 20050093200 are hereby incorporated by reference to the extent not incompatible herewith.
There may be several motivations for drawing gel-spun polyethylene filaments and yarns. The end-use applications may require low filament denier or low yarn denier. Low filament deniers are difficult to produce in the gel spinning process. Solutions of UHMWPE are of high viscosity and may require excessive pressures to extrude through small spinneret openings. Hence, use of spinnerets with larger openings and subsequent drawing may be a preferable approach to producing fine denier filaments. Another motivation for drawing may be a need for high tensile properties. Tensile properties of gel-spun polyethylene filaments generally improve with increased draw ratio if appropriately conducted. Yet another motivation for drawing may be to produce a special microstructure in the filaments that may be especially favorable for particular properties, for example, ballistic resistance.
Dynamic mechanical analysis (DMA) is the technique of applying a dynamic stress or strain to a sample and analyzing the response to obtain mechanical properties such as storage modulus (E′), loss modulus (E″) and damping or tan delta (δ) as a function of temperature and/or frequency. An introductory description of DMA as applied to polymers has been presented by K. P. Menard in “Encyclopedia of Polymer Science and Technology”, Volume 9, P. 563–589, John Wiley & Sons, Hoboken, N.J., 2004. Menard indicates that DMA is very sensitive to molecular motions of polymer chains and is a powerful tool for measuring transitions in such motions. Temperature regions in which transitions in molecular motion occur are marked by departure of E′, E″ or tan δ from base line trends and are variously termed “relaxations” and “dispersions” by investigators. DMA studies of many polymers have identified three temperature regions associated with dispersions designated alpha (α), beta (β) and gamma (γ).
Khanna et al., Macromolecules, 18, 1302–1309 (1985), in a study of polyethylenes having a range of densities (linearity), attributed the α-dispersion to molecular motions of chain folds, loops, and tie molecules at the interfacial regions of crystalline lamellae. The intensity of the α-dispersion increased with increasing lamellar thickness. The β-dispersion was attributed to molecular motions in the amorphous interlamellar regions. The origin of the γ-dispersion was not clear but was suggested to involve mostly the amorphous regions. Khanna et al. note that K. M.Sinnott, J. Appl Phys., 37, 3385 (1966) proposed that the γ-dispersion was due to defects in the crystalline phase. In the same study, Khanna et al. associated the α-dispersion with transitions in molecular motions above about 5° C., the β-dispersion with transitions between about −70° C. and 5° C., and the γ-dispersion with a transition between about −70° C. and −120° C.
R. H. Boyd, Polymer, 26, 323 (1985) found that as crystallinity increased, the γ-dispersion tended to broaden. Roy et al., Macromolecules, 21(6), 1741 (1988) in a study of UHMWPE films gel-cast from very dilute solution (0.4% w/v) found that the γ-dispersion disappeared when the sample was hot drawn in the solid state in the region beyond 150:1. K. P. Menard (citation above) noted a correlation between toughness and the β-dispersion.
U.S. Pat. No. 5,443,904 suggested that high values of tan δ in the γ-dispersion could be indicative of excellent resistance to high speed impact, and that high peak temperature of the loss modulus in the α-dispersion was indicative of excellent physical properties at room temperature.
It should be noted that DMA instruments may be of different types and have different modes of operation that may effect the results obtained. A DMA instrument may impose a forced frequency on the sample or the instrument may be of a free resonance type. A forced frequency instrument may be operated in different modes (stress controlled or strain controlled). Since most dynamic mechanical analyses of polymers are run over a range of temperatures where the static force in the sample may change as a result of sample shrinkage, thermal expansion, or creep, it is necessary to have some mechanism to adjust the sample tension when temperature is changed. The DMA instrument may be run with a constant static force set at the start of the test to a value greater than the maximum dynamic force observed during the test. In this mode, the sample is prone to elongate as it softens on heating, resulting in a possible change in morphology. Alternatively, the DMA instrument may automatically control and adjust the static force to be a certain percent greater than the dynamic force. In this mode, the sample elongation and morphology change during the test are minimized and the DMA properties measured will be more representative of the original sample before heating.